feat(video): add video upload with remux/transcode playback

Add video as a media kind in photos: streaming upload with per-kind
limits, ffprobe metadata, poster frame reusing the image variant
pipeline, and a playback object (remux when web-compatible, else
h264/yuv420p/aac transcode). Video jobs run on a separate pool with a
long lease. Frontend gains a video viewer, grid badge/duration and
video upload.

backend/internal/models/models.go

@@ -71,10 +71,13 @@ type Photo struct {
 	ThumbKey         string     `json:"-"`
 	MediumKey        string     `json:"-"`
 	PreviewKey       string     `json:"-"`
+	PlaybackKey      string     `json:"-"`
 	OriginalFilename string     `json:"original_filename"`
 	Size             int64      `json:"size"`
 	Width            int        `json:"width"`
 	Height           int        `json:"height"`
+	Kind             string     `json:"kind"`
+	DurationMS       int64      `json:"duration_ms,omitempty"`
 	MimeType         string     `json:"mime_type"`
 	TakenAt          *time.Time `json:"taken_at,omitempty"`
 	Latitude         *float64   `json:"latitude,omitempty"`
@@ -92,6 +95,7 @@ type Photo struct {
 	ThumbURL         string     `json:"thumb_url,omitempty"`
 	MediumURL        string     `json:"medium_url,omitempty"`
 	PreviewURL       string     `json:"preview_url,omitempty"`
+	PlaybackURL      string     `json:"playback_url,omitempty"`
 	OriginalURL      string     `json:"original_url,omitempty"`
 	DownloadURL      string     `json:"download_url,omitempty"`
 }